1. Field of the Invention
The present invention relates primarily to a process and apparatus used in the manufacture of multi-layered printed circuit boards and more specifically, to improvements useful in plasma desmear etching of such boards.
2. Prior Art
Printed circuit board technology is an old and well-known art in the electronics industry. The space saving, low cost and productivity advantages of printed circuit boards for electronic circuitry are well-known benefits enjoyed by virtually all manufacturers of electrical and electronic equipment. As with other areas in electronics technology, the state of the art in printed circuit board design and manufacture has made great strides over the years. As a result, the size of circuit boards has decreased, the density has increased, reliability has improved and productivity has been greatly enhanced. Many of these state of the art improvements in printed circuit board technology relate to the more recent use of multi-layered laminated printed circuit boards.
Although multi-layered printed circuit boards are highly advantageous in that they are capable of providing greatly enhanced circuit density within a given printed circuit board volume, they also present a more complex engineering problem insofar as the manufacture of the board is concerned. For example, the manner of electrically interconnecting the appropriate portions of the circuit layers of the multi-layered printed circuit board is one such complication. This complexity has been readily solved by providing component pin receiving holes that extend through the various layers of the board to provide a means of selectively interconnecting the circuits of the board which are otherwise isolated from one another by intermediate layers of a dielectric material such as an acrylic. Such component pin holes in a multi-layered printed circuit board are very critical to the goal of achieving required performance of the circuit and reliability of the board.
It is very important that the holes comprising the various layers of a multi-layered printed circuit board be of the proper dimension and shape. The metal portions within such holes should be accessible for good contact with the component pin to be installed therein for proper electrical conductivity therebetween without inadvertent insulation as a result of dielectric material. Extraneous dielectric material may result from the circuit board production process and is commonly referred to in the industry as the smear. In the earlier prior art of multi-layered printed circuit board construction, each circuit board layer of the laminated structure was made principally of an epoxy dielectric material. The process of removing the smear from each component hole of the circuit board, a process commonly referred to as desmearing, was readily accomplished by applying appropriate acid such as sulfuric acid to each of the mechanically drilled holes.
This type of acid treatment served a dual purpose. Not only did it remove the smear from the hole but it also performed, to a certain extent, an etching of the intermediate dielectric board layers to measurably increase the diameter of those layers along the inner surface of the hole. As a result, individual annular metalized portions of the respective circuit board layers, which were not affected or affected less by the acid, extended radially into the hole to a greater extent on the order of several thousandths of an inch. Such metalized portions assure a biting metal-to-metal contact between the circuit of each board layer and the metal pin of the component installed into the board holes.
In the more recent prior art of multi-layered printed circuit board manufacture, the substances used for manufacture of the dielectric portion of the multi-layered printed circuit board have been changed to more exotic modern materials such as acrylic, polyimide, Kapton, and Kevlar. These new materials have become particularly advantageous for multi-layered printed circuit board manufacture because they can withstand a more severe environment than epoxy boards. For example, they are far more adapted for use in long term, high temperature applications. Furthermore, many modern packaging requirements particularly for the military and missile applications, and in airborne and satellite applications, require compliance with much more stringent military specifications. The use of higher densities in a multi-layered printed circuit board is often more readily accommodated in flexible structures for easier design and installation within small volumes. In such flexible applications particularly it has been found highly advantageous to use a multi-layer flexible circuit laminated between polyimide glass sections. Twenty layers or more of flex circuitry with board sizes of 24.times.24 inches or greater can be readily manufactured in such flexible configurations.
Unfortunately, these new exotic materials including polyimide as an example, do not react satisfactorily to acid desmearing and do not become etched to provide the aforementioned advantages derived in using acid on epoxy dielectric materials. Consequently, alternative desmearing and etching techniques have had to be developed.
Fortunately, a new technique has been developed recently which except for a substantial disadvantage to be described hereinafter, provides a viable means for desmearing and etching the holes in multi-layered printed circuits which use the aforementioned modern, more exotic dielectric materials. This recent innovation is commonly referred to as a plasma desmear etcher. In such plasma systems, each circuit board is placed between a pair of parallel plasma plates which are separated from one another by approximately 3 inches and which extend over the entire surface area of the printed circuit board. This is accomplished within an airtight chamber in which the air is evacuated by a pump and replaced with a known mixture of selected gases such as oxygen and freon. A high power radio frequency electric field is then applied to the plates to generate a plasma between the plates. In one typical example the electric field is at a frequency of 4.5 MHz. at a power of 2,200 watts. The plasma around the printed circuit board between the two plates produces a plurality of discharge sparks that occur between the plates and through the predrilled apertures in the printed circuit board. These discharges through the apertures desmear the apertures and etch back the dielectric material despite the use of acrylics and polyimides as well as epoxy resin dielectrics. Thus, the plasma desmear etcher replaces the prior art acid desmearing process and works well with the newer more exotic materials that are resistant to acid desmearing and etching.
However, there is one substantial problem associated with this plasma desmear etcher process which prevents the plasma innovation from being absolutely effective in all respects. This disadvantage relates to the difficulties in controlling the plasma so that the discharge is everywhere consistent and homogeneous along the entire surface of the printed circuit board. The negative effect of this inconsistency is to make the etching of the printed circuit board apertures dependent upon circuit board location. Accordingly, some apertures may be etched beyond the required specifications and some located in a different position on the board may not be adequately etched. Furthermore, in order to assure a minimum level of etching of all apertures irrespective of the aperture location on the board, those portions of the board which receive a more powerful discharge tend to be scorched and over-etched. The inconsistency of plasma desmearing and etching is therefore more likely to reduce the printed board yield during manufacture, increase the cost of manufacture, decrease the possibilities for long term reliability and increase the risk of providing the user with a faulty board.
In military applications where the reliability of the printed circuit board is especially critical, it is common to provide an extra portion of the board for production purposes only. This extra portion or test tab is used as a test vehicle to indicate whether the desmear and etching process during board production has been satisfactory. Such test vehicle portions of the board are typically disposable and are located along the outer perimeter of the board in selected locations. However, when plasma desmear consistency is lacking, even a satisfactorily tested disposable board portion may not provide an accurate indication of the desmeared and etched condition of the board at some portion thereof distant from the test vehicle portion of the board. The present invention is designed to be used with plasma desmear and etching systems to overcome the aforementioned disadvantage with respect to such systems. The applicants know of no other prior art device, system, apparatus or materials which have heretofor been available for overcoming the aforementioned disadvantage.